1. Field of the Invention
The present invention relates to a solder composition substantially free from Pb and a soldered article therewith. Particularly, the present invention relates to a Pb-free solder composition suitable for soldering a conductor formed on a brittle substrate, and to a soldered article such as a soldered substrate therewith.
2. Description of the Related Art
When soldering is conventionally carried out onto a conductor formed on a glass substrate, a Snxe2x80x94Pb type solder composition having a low tensile modulus is generally used. However, since the conventional Snxe2x80x94Pb type eutectic solder composition contains poisonous Pb, there have been an increasing number of cases in which the use is restricted.
Recently, a Pb-free solder composition mainly composed of Sn has also been used in consideration of the burden on the environments. However, when the soldering with the composition is carried out on to a conductor formed on a fragile substrate such as a glass substrate, a thermal stress generated in the course of the soldering will sometimes damage the substrate. This problem is caused by the fact that the solder usually has a large tensile modulus.
Furthermore, a higher and higher heat-resistant temperature has been required for a soldered article in recent years. For example, when the conventional Snxe2x80x94Pb eutectic solder composition or the Pb-free solder composition mainly composed of Sn is applied for soldering an article and the soldered article is exposed to a high temperature, the solder composition is melted, incurring solder flowing, coming-off of terminals, electrode erosion, disconnection, etc. These are problems of so-called solder heat resistance-related defects (that is, defects related to the heat resistance of a solder).
Accordingly, it is an object of the present invention to solve such problems, to provide a Pb-free solder composition with which damage on a fragile substrate such as a glass substrate can be restricted when the soldering with the composition is carried out on to a conductor formed on the substrate and which has an excellent heat resistance, and to provide a soldered article therewith.
In order to attain the above-described purposes, one aspect of the present invention is a Pb-free solder composition comprising: Bi as a first metallic element; a second metallic element which can form a binary eutectic at a ratio of not more than about 9.9 parts by weight of the second metallic element based on not less than about 90 parts by weight of the above-described first metallic element; and a third metallic element, wherein the first metallic element is not less than about 90% by weight, and the third metallic element is from about 0.1% to 3.0% by weight, both based on the whole solder composition.
In the Pb-free solder composition according to the present invention, it is especially preferable that the first metallic element is not less than about 94.5% by weight, and the second metallic element is from about 0.15% to 3.0% by weight of the whole solder composition.
Furthermore, it is preferable that the Pb-free solder composition according to the present invention does not contain a low melting-point eutectic with a solidus temperature of less than about 200xc2x0 C.
Furthermore, the third metallic element in the Pb-free solder composition according to the present invention is preferably at least one element selected from the group consisting of Sn, Cu, In, Sb and Zn.
Furthermore, it is preferable that the second metallic element is at least one element selected from the group consisting of Ag, Cu and Zn. It is more preferable that the second metallic element is from about 0.1% to 9.9% by weight of the above-described whole solder composition.
In the Pb-free solder composition according to the present invention, it is especially preferable that the above-described second metallic element is Ag, the above-described third metallic element comprises at least one selected from the group consisting of from about 0.1% to 0.5% by weight of Sn, from about 0.1% to 0.3% by weight of Cu, from about 0.1% to 0.5% by weight of In, from about 0.1% to 3.0% by weight of Sb, and from about 0.1% to 3.0% by weight of Zn, based on the whole solder composition.
Furthermore, in the Pb-free solder composition according to the present invention, it is preferable that the above-described third metallic element further comprises at least one element selected from the group consisting of Ge and P, and that this additional third metallic element is from about 0.01% to 0.1% by weight of the whole solder composition.
Another aspect of the present invention provides a Pb-free soldered article comprising: a substrate having a conductive pattern on the surface; and a Pb-free solder composition according to the present invention placed so as to be electrically and mechanically connected to the conductive pattern.
The above-described substrate may be a glass substrate having an electrode pattern on its surface, wherein a lead wire is connected to the electrode via the above-described Pb-free solder composition.